Printing machines include a print unit where an image is imparted on a printable substrate and a transfer unit that carries the substrate along a process path to and through the print unit. Print units may include direct printing devices such as inkjet units. Inkjet printing units include a plurality of print heads that emit drops of ink onto the printable substrate in a tightly controlled manner to create an image.
With print units such as inkjet print units, it is desirable for image quality that the distance between the printable substrate's upper surface and the face of the inkjet print heads is kept consistent. The distance can be very small, on the order of a 0.010″−0.040″. The printable substrate must be maintained in a uniformly flat orientation as it travels under the print heads in order to prevent the substrate from engaging the print heads. The print heads can be damaged if the printable substrate engages them.
The transfer device can include a hold down device that restrains the printable substrate on a transfer belt. The hold down device also attempts to flatten the printable substrate before the printable substrate travels under the print heads. The substrate can have thickness variations caused by waves or ripples in the substrate. Various methods of holding and flattening the substrate have been used including vacuum hold downs. However, increasing the vacuum level to try and flatten and smooth out a substrate can lead to increased friction between the transport belt and the underlying surface over which the transport belt passes. This can lead to operational and maintenance problems.
Furthermore, substrates, such as envelopes, which have multiple layers and an unsecured flap, are especially difficult to hold in a uniformly flat orientation. The vacuum belt acts primarily on the bottom layer of the envelope and does not smooth out the top layer which is adjacent to the print heads. Also, the high speed associated with envelope printing further complicates the task of retaining and transporting the envelope such that the top surface remains within a predetermined distance from the print heads.
Accordingly, it would be desirable to provide a transfer device that transports a printable substrate and flattens its surface prior to entering the print unit.